1. Field of the Invention
The present invention is related to a technique capable of improving image qualities when entered digital images are outputted to printers so as to be printed, or outputted to display apparatus so as to be displayed.
2. Description of the Related Art
Recently, since digital information is communicated which is originated from digital cameras, scanners, electronic documents, RTML documents, and the like, strong demands are made to display/print digital images (will be simply referred to “images” hereinafter) in high image qualities. While these images contain a large number of image quality deterioration components known as blurring components, jaggy components, and noise components, when such images are displayed and/or printed in high image qualities, various image quality improving processing are required. These image quality improving processing contain, for instance, corrections of blurring portions, antialiasing corrections (namely, corrections of jaggy portions), noise eliminations, and the like. For instance, there are some cases that blurring components, jaggy components, and noise components are contained in images of digital cameras, which are caused from optical limitations. Also, in such a case of JPEG-compressed images which are utilized many times, noise caused by image processing may be further produced. This noise is called as mosquito noise and/or block noise. As a result, processing for removing the above-explained noise is required. Also, in character recognizing fields and image diagnosing (medical image) fields, such processing capable of removing noise components and blurring components are required before recognizing is performed.
As techniques capable of improving image qualities, there are an edge enhancement processing, an antialiasing processing, a noise removing processing, and so on. A large number of technical ideas so as to these image quality improving processing have been conventionally disclosed. The below-mentioned technical ideas are recited as one example.
As the edge enhancement processing, for example, JP-A-7-66977 describes such an edge emphasizing method that while both a minimum value and a maximum value within a block are detected, a pixel value is limited within a range of these minimum/maximum values and is converted by employing a quadratic differential filter so as to execute the edge emphasizing. Also, JP-A-2000-200349 describes such a method for calculating a predicted ideal edge enhancement pixel value in such a manner that either a maximum value or a minimum value is selected from a plurality of images which have been thinned by different thinning intervals.
As the antialiasing processing (correction of jaggy portions), for example, JP-A-5-276382 describes such a method that a dot pattern within a predetermined range which contains an interesting dot of a binary image is compared with a predetermined pattern, and data of dots is changed based upon a comparison result so as to produce either a dropped dot or a projected dot. Also, JP-A-7-288693 describes such a method that a triangle projected from a jaggy portion is moved to a concave portion so as to smooth an edge, and then, this smoothed edge is filtered by a low-pass filter.
Furthermore, as the noise removing processing, for example, JP-A-10-229546 discloses such a noise removing method. That is, a block noise portion is selected based upon an averaged variation of pixel values and a variation of block boundary pixels with respect to pixels located around a boundary of compressed blocks when being coded. Then, a constant which is obtained based upon these variations is superimposed to the selected block noise portion, and the averaged variation is made coincident with the block boundary variation value so as to reduce block noise, and further to perform the edge enhancement processing. Also, as described in JP-A-11-085961, there is another noise reducing method. That is, with respect to such a pixel that existing probability of line-shaped shadows is high as to an X-ray medical image, a smoothing effect of a filter is suppressed to a lower smoothing effect, so that lowering of contrast is prevented. On the other hand, with respect to such a pixel that existing probability of line-shaped shadows is low and also probability corresponding to background noise is high, the a smoothing effect of the filter is increased so as to effectively reduce noise.
However, when the above-described conventional image quality improving processing is employed, the problems as to the image qualities can be individually solved. However, in such a case that image qualities are improved in a comprehensive manner, while the blurring portion, the jaggy portion, and the noise portion are separately recognized, the respective different methods are used at the same time, or in a sequential manner in order to highly improve these image qualities. As a result, there are some problems that the processing may become complex and also, the resulting image qualities are not made constant. Also, in such a case that the different methods are used in a combined manner, there is another problem that a stepped portion formed at a boundary portion where the processing are switched is changed in an unnatural condition, and thus becomes conspicuous. Furthermore, when the respective blurring/jaggy/noise portions are recognized, these portions are recognized in an erroneous manner and processed in erroneous methods, there is another problem. That is, the image qualities are further deteriorated, as compared with the deterioration portions of the original image.
As another aspect related to the image quality improving techniques, image enlarging techniques (namely, resolution converting techniques) are required in such a case that digital images produced from the above-described digital cameras, scanners, electronic documents, and HTML documents are enlarged to output the enlarged images, and in addition, when such media having different resolution is coupled to each other, e.g., in HDTV (high definition television) system, an NTSC television system, an electronic still camera, a medical image system, and a printing image system are coupled to each other. Images own various different resolution, and on the other hand, output apparatus such as printers for printing processing and display apparatus for display processing have resolution specific thereto. As a consequence, resolution conversions are necessarily required among mutual resolution. Concretely speaking, for instance, in order that an image having resolution of 75 dpi which is viewed on a display is outputted to a printer having resolution of 600 dpi, an enlarging factor of 8 times is required. Since these conventional enlarging process methods are employed, an enlarging processing can be performed in high image quality, while an original image is faithfully maintained.
However, when an image is enlarged, in such a case that this image contains the above-explained deterioration components (namely, blurring component, jaggy component, and noise component), even if an edge of this image may be smoothly enlarged by an enlarging algorithm, the following case happens to occur. That is, lowering of an image quality which is caused by directly enlarging these deterioration components may become conspicuous, while neglecting the edge smoothing effect. In such a case, the effect by the enlarging algorithm cannot be sufficiently accepted. Also, even when the conventional image quality improving processing are used as the pre-processing, and the image is enlarged after the deterioration components have been previously removed, if the matching characteristic cannot be established between this pre-processing and the image enlarging processing, then the effects cannot be sufficiently accepted.
When the above-described problems of the conventional techniques are summarized in the comprehensive manner, the blurring portion, the jaggy portion, and the noise portion cannot be processed by employing a single method in the conventional technique. To this end, for example, while the blurring portion, the jaggy portion, and the noise portion are separately recognized, it is so required that the different methods from each other are used so as to improve the image quality in high precision. Therefore, there are such problems that the processing become complex and the resulting image qualities are not made constant. Also, in such a case that the different methods are used in a combined manner, there is another problem that a stepped portion formed at a boundary portion where the processing are switched is changed in an unnatural condition, and thus becomes conspicuous. Furthermore, when when the respective blurring/jaggy/noise portions are recognized, these portions are recognized in an erroneous manner and are processed in erroneous methods, there is another problem. That is, the image qualities are further deteriorated, as compared with the deterioration portions of the original image. Also, since these image quality improving processing are not performed under such an initial condition that the image enlarging processing is thereafter performed, there is a further problem. That is, the image quality cannot be improved in order that the image can be improved in the higher image quality by the enlarging algorithm.